Dispersion-compensating module

ABSTRACT

The present invention relates to a dispersion-compensating module having a structure which compensates for the dispersion of an optical transmission line in a wavelength band of 1.55 μm and adjusts loss fluctuations among wavelengths in the wavelength band of 1.55 μm into an appropriate range. The module comprises a structure adapted to be installed in an already installed optical fiber transmission line, and has a loss slope with a polarity opposite to that of the optical fiber transmission line in the wavelength band of 1.55 μm. An example of the module comprises a dispersion-compensating optical fiber as a dispersion-compensating device, and an optical fiber doped with a transition metal element as a loss-equalizing device. Consequently, the loss fluctuations among individual wavelengths in the whole transmission line including the dispersion-compensating module are adjusted by the loss-equalizing device in the dispersion-compensating module so as to fall within an appropriate range.

RELATED APPLICATIONS

This is a Continuation-In-Part application of a continuation applicationfiled on Apr. 2, 2002 from patent application Ser. No. 09/452,103 filedon Dec. 2, 1999, now U.S. Pat. No. 6,404,950.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dispersion-compensating module whichimproves the transmission quality of large-capacity, high-speed opticaltransmission systems of WDM (Wavelength Division Multiplexing) type.

2. Related Background Art

A WDM type optical transmission system is a system which transmits aplurality of signal light components within a 1.55-μm wavelength band(1.53 μm to 1.57 μm) by way of an optical fiber transmission linenetwork, and enables large-capacity, high-speed optical communications.This optical transmission system comprises an optical amplifier forcollectively optically amplifying a plurality of signal lightcomponents, and the like, in addition to an optical fiber line which isa transmission medium. In such WDM communications, various studies anddevelopments are under way in order to enable communications withfurther larger capacity and higher speed.

One of important subjects to be studied concerning the opticaltransmission line is reduction of dispersion in its signal wavelengthband. Namely, each signal light component has a certain bandwidth eventhough it is monochromatic. As a result, when dispersion occurs in thewavelength band of the signal light in the optical transmission line,the signal light having reached a receiving station by way of theoptical transmission line after being sent out from a transmittingstation deforms its waveform, thereby deteriorating its reception.Therefore, in the signal light wavelength band, it is desirable that thedispersion of the optical transmission line be as small as possible.

However, standard single-mode optical fibers (hereinafter referred to asSMF), having a zero-dispersion wavelength in a 1.3-μm wavelength band,already installed as an optical transmission line have a dispersion ofabout 16 ps/nm/km in a wavelength band of 1.53 μm to 1.57 μm which isused in WDM communications. Many of already installed opticaltransmission lines are constituted by such an SMF. Therefore, adispersion-compensating module is disposed within a repeater in order tocompensate for the dispersion of the optical transmission line, whilemaking use of such an already installed optical transmission line.

With respect to the dispersion over the whole length of the opticaltransmission line to be compensated for, the dispersion-compensatingmodule generates a dispersion having an opposite polarity withsubstantially the same absolute value. Specifically, thedispersion-compensating module comprises a dispersion-compensatingoptical fiber having a dispersion with a polarity opposite to that ofthe dispersion of the optical transmission line (including SMF), andcompensates for the dispersion of the optical transmission line(including SMF) by adjusting the dispersion-compensating optical fiberto an appropriate length. Also, for reducing the size of thedispersion-compensating module, it is a common practice to wind thedispersion-compensating optical fiber into a coil having a smalldiameter.

SUMMARY OF THE INVENTION

The inventors have studied conventional dispersion-compensating modulesand, as a result, have found out the following problems.

Recently, in optical amplifiers, the wavelength bandwidth of signallight which can collectively be optically amplified has been expanded,and gain deviations in optically amplifiable signal wavelength bandshave been reduced in order to improve the versatility thereof. On theother hand, inter-wavelength loss deviations (fluctuations in loss amongwavelengths) occurring in the optical transmission line in the signallight wavelength band are too large to neglect. Also, sinceinter-wavelength loss deviations in the signal light wavelength bandoccur to a certain extent in a dispersion-compensating optical fiber aswell, it is necessary to improve loss deviations in the wholetransmission line including the conventional dispersion-compensatingmodules.

When a plurality of stages of conventional dispersion-compensatingmodules and optical amplifiers are disposed on such an opticaltransmission line; even if a plurality of signal light components sentout from a transmitting station exhibit no inter-wavelength opticalpower deviation at the time when sent out, they will generate an opticalpower deviation, due to inter-wavelength loss deviations, while they arepropagating through the optical transmission line and conventionaldispersion-compensating modules, and the optical power deviation isexpanded by optical amplifiers having a small gain deviation. Since theplurality of signal light components reaching the receiving stationconstitute signal light in which inter-wavelength optical powerdeviations are accumulated, the signal light power may become weakdepending on wavelength, thereby generating reception errors.

In order to overcome the above-mentioned problems, it is an object ofthe present invention to provide a dispersion-compensating module havinga structure which compensates for the dispersion of optical transmissionlines in a 1.55-μm wavelength band (1.53 μm to 1.57 μm) and adjusts lossfluctuations among signal light components into an appropriate range.

The dispersion-compensating module according to the present invention isgenerally installed between repeaters together with an opticalamplifier, whereas the object to be compensated for thereby is an SMFwhich is laid between a transmitting station and a receiving station,between repeater stations, between the transmitting station and arepeater station, or between a repeater station and the receivingstation. For being installed on an already installed optical fibertransmission line, the dispersion-compensating module comprises an inputend for capturing signal light propagating through the optical fibertransmission line and an output end for sending out the signal lightinto the optical fiber transmission line, has a positive loss slope in a1.55-μm wavelength band, and further comprises a structure for allowingthe dispersion generated in a predetermined length of the optical fibertransmission line to be compensated for and loss deviations amongindividual wavelengths to be adjusted into an appropriate range.

Specifically, the dispersion-compensating module according to thepresent invention comprises a dispersion-compensating device and aloss-equalizing device. The dispersion-compensating device compensatesfor the dispersion of the above-mentioned optical fiber transmissionline in the 1.55-μm wavelength band. Also, for compensating for thewavelength dependence of loss in at least the above-mentioned opticalfiber transmission line and dispersion-compensating device, theloss-equalizing device adjusts the total loss slope of thedispersion-compensating module such that the loss deviations amongindividual wavelengths in the 1.55-μm wavelength band caused bypropagation in the optical fiber transmission line anddispersion-compensating device falls within the appropriate range.

In this specification, “loss slope” refers to the gradient of a graphindicating the wavelength dependence of transmission loss. Also, theabove-mentioned optical fiber transmission line is an SMF having azero-dispersion wavelength in a 1.3-μm wavelength band; and, letting Lbe the length of the above-mentioned SMF, and α be the absolute value ofa permissible manufacturing error, the loss slope (dB/nm) of thedispersion-compensating module in the 1.55-μm wavelength band is greaterthan 0 but not greater than 0.000175×L+α. In general, when an SMF havinga zero-dispersion wavelength in a 1.3-μm wavelength band is employed asan optical fiber transmission line, the loss slope per unit length ofthe SMF is about −0.000175 (dB/nm/km =dB/(nm·km)). Therefore, the lossslope of the dispersion-compensating module is ideally +0.000175×L whenan SMF having a length of L is concerned. In practice, however, sincethe manufacturing error α cannot be neglected, the loss slope (dB/nm) ofthe dispersion-compensating module in the 1.55-μm wavelength band isgreater than 0 but not greater than 0.000175×L+α.

The loss-equalizing device includes an optical fiber, comprising a coreregion doped with a transition metal element and a cladding regiondisposed at an outer periphery of the core region, in which a singlemode is secured in the 1.55-μm wavelength band. Preferred examples ofthe transition metal element include Cr and Co, and the amount ofcompensation of loss in the loss-equalizing device can be adjusted whenthe kind of the transition metal element and the amount of additionthereof are appropriately regulated.

The above-mentioned loss-equalizing device may include an optical fiberformed with a long-period grating in which a propagation mode and aradiation mode are coupled to each other. Alternatively, theabove-mentioned dispersion-compensating device may be an optical devicehaving, as the above-mentioned loss-equalizing device, a long-periodgrating in which a propagation mode and a radiation mode are coupled toeach other. In each of these configurations, the long-period grating,which is the loss-equalizing device, enables loss deviations amongindividual wavelengths to be adjusted in the whole optical transmissionline without increasing the transmission loss of the wholedispersion-compensating module. In particular, in the configuration inwhich the long-period grating, which is the loss-equalizing device, isformed in the optical fiber acting as the dispersion-compensatingdevice, the dispersion-compensating device does not have a connectingportion which may yield loss. Consequently, it is not necessary to takeaccount of influences of transmission loss in the connecting portion,whereby loss fluctuations among individual wavelengths can be adjustedmore easily. Here, as explicitly shown in U.S. Pat. No. 5,703,978, thelong-period grating is a grating which induces coupling (mode coupling)between a core mode and a cladding mode which propagate through anoptical fiber, and is clearly distinguished from a short-period gratingfor reflecting light centered at a predetermined wavelength. Also, inthe long-period grating, for yielding a strong power conversion from thecore mode to the cladding mode, the grating period (pitch) is set suchthat the optical path difference between the core mode and the claddingmode becomes 2π. As a consequence, since the long-period grating acts soas to couple the core mode to the cladding mode, the core modeattenuates over a narrow band centered at a predetermined wavelength(hereinafter referred to as “loss wavelength”).

The above-mentioned loss-equalizing device can also be realized byconnecting respective end portions of a pair of optical fibers byfusion. In this case, the fused portion of the pair of optical fibersfunctions as the loss-equalizing device.

Preferably, the pair of optical fibers are connected by fusion at thefused portion while their optical axes are shifted from each other. Thisfused portion may also be realized by connecting the pair of opticalfibers by fusion while their core regions are bent. Also, when a pair ofoptical fibers each having a core region with an outside diameterexpanding toward the opposed portion is connected to each other byfusion, the fused portion functions as the loss-equalizing device.

The loss-equalizing device may include a fiber coupler, or an opticalfiber bent at one or more parts thereof. A desirable loss wavelengthcharacteristic can be obtained in each of these cases.

Further, the loss-equalizing device may include any one of a slant typefiber grating, which comprises an optical fiber and a grating formed inthe optical fiber and being inclined at a predetermined angle withrespect to an optical axis of the optical fiber, and a dielectricmultilayered filter. The loss-equalizing device preferably have avaliable wavelength chanracteristic, and includes, for example, avariable loss-equalizing device having a planar waveguide or a variableloss-equalizing device with MEMS (Micro Electro Mechanical Systems).

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a schematic configuration of a first embodimentof the dispersion-compensating module according to the presentinvention;

FIGS. 2A to 2E are graphs showing relationships between loss andwavelength in respective parts in each of dispersion-compensatingmodules according to first, second, fourth, fifth, and sixthembodiments;

FIG. 3A is a graph showing the loss wavelength characteristic of astandard single-mode optical fiber having a zero-dispersion wavelengthin a 1.3-μm wavelength band, whereas FIG. 3B is a graph showing, to alarger scale, the loss wavelength characteristic of the single-modeoptical fiber in the vicinity of a 1.5-μm wavelength band in the graphshown in FIG. 3A;

FIG. 4A is a graph showing an example of the loss wavelengthcharacteristic of a loss-equalizing optical fiber whose core region isdoped with Co element (transition metal element), whereas FIG. 4B is agraph showing, to a larger scale, the loss wavelength characteristic ofthe loss-equalizing optical fiber in the vicinity of the 1.5-μmwavelength band in the graph shown in FIG. 4A;

FIG. 5 is a view showing a schematic configuration of a secondembodiment of the dispersion-compensating module according to thepresent invention;

FIG. 6 is a graph showing an example of the loss wavelengthcharacteristic of a long-period grating;

FIG. 7 is a view showing a schematic configuration of a third embodimentof the dispersion-compensating module according to the presentinvention;

FIGS. 8A to 8D are graphs showing relationships between loss andwavelength in respective parts in the dispersion-compensating moduleaccording to the third embodiment shown in FIG. 7;

FIG. 9 is a view showing a schematic configuration of a fourthembodiment of the dispersion-compensating module according to thepresent invention;

FIGS. 10A to 10C are views for explaining specific configurations of thefused portion as a loss-equalizing device in the dispersion-compensatingmodule according to the fourth embodiment shown in FIG. 9;

FIG. 11 is a graph showing an example of the loss wavelengthcharacteristic of the fused portion shown in FIGS. 10A to 10C;

FIG. 12 is a view showing a schematic configuration of a fifthembodiment of the dispersion-compensating module according to thepresent invention;

FIG. 13 is a view showing a schematic configuration of a sixthembodiment of the dispersion-compensating module according to thepresent invention; and

FIG. 14 is a view showing a schematic common configuration of seventh totenth embodiments of the dispersion-compensating module according to thepresent invention; and

FIGS. 15A to 15F are views for explaining specific configurations ofloss-equalizing devices in the dispersion-compensating modules accordingto the seventh to tenth embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the dispersion-compensating moduleaccording to the present invention will be explained with reference toFIGS. 1, 2A to 4B, 5 to 7, 8A to 8D, 9, 10A to 10C, 11 to 14 and 15A to15F. In the explanation of the drawings, constituents identical to eachother will be referred to with numerals or letters identical to eachother without repeating their overlapping descriptions.

First Embodiment

To begin with, a first embodiment of the dispersion-compensating moduleaccording to the present invention will be explained. FIG. 1 is a viewshowing a schematic configuration of the dispersion-compensating moduleaccording to the first embodiment. This drawing shows, in addition tothe dispersion-compensating module 10 according to this embodiment, arepeater 1 disposed upstream of the dispersion-compensating module 10,and an optical transmission line 2 between the repeater 1 and thedispersion-compensating module 10.

The dispersion-compensating module 10 according to this embodiment hasan input end 10 a and an output end 10 b, and is disposed while in astate where a dispersion-compensating device and a loss-equalizingdevice are optically connected to each other in the optical path betweenthe input end 10 a and the output end 10 b. In particular, thedispersion-compensating module 10 is constituted by adispersion-compensating optical fiber 11, as the dispersion-compensatingdevice, and an optical fiber 12 doped with a transition metal element,as the loss-equalizing device, which are connected to each other byfusion at a connecting portion 13.

The dispersion-compensating optical fiber 11 is an optical device whichcompensates for the chromatic dispersion in the WDM signal wavelengthband of the optical transmission line 2 into which thedispersion-compensating module 10 is inserted. On the other hand, thetransition-metal-element-doped optical fiber 12 is an optical fiber,basically comprising a core region and a cladding region disposed at theouter periphery of the core region, in which a transition metal elementsuch as Cr element, Co element, or the like is added at least into thecore region. When the kind and amount of the transition metal elementadded to the core region are appropriately selected, then the losswavelength characteristic of the optical fiber 12 itself is adjusted soas to compensate for wavelength-dependent loss deviations of the opticaltransmission line 2 and dispersion-compensating optical fiber 11. As aconsequence, the total loss fluctuation in the signal wavelength band ofthe optical transmission line 2 provided with thedispersion-compensating module 10 decreases.

FIGS. 2A to 2E are graphs showing relationships between transmissionloss and wavelength in respective parts in the dispersion-compensatingmodule according to first embodiment. In particular, FIG. 2A is a graphshowing the relationship between transmission loss and wavelength in awavelength band of 1.53 μm to 1.57 μm in the optical transmission line 2employing an SMF having a zero-dispersion wavelength in a 1.3-μmwavelength band. FIG. 2B is a graph showing the relationship betweentransmission loss and wavelength in the wavelength band of 1.53 μm to1.57 μm in the dispersion-compensating optical fiber 11 acting as thedispersion-compensating device. FIG. 2C is a graph showing therelationship between transmission loss and wavelength in the wavelengthband of 1.53 μm to 1.57 μm in the transition-metal-element-doped opticalfiber 12 acting as the loss-equalizing device. FIG. 2D is a graphshowing the relationship between transmission loss and wavelength in thewavelength band of 1.53 μm to 1.57 μm in the wholedispersion-compensating module 10. FIG. 2E is a graph showing therelationship between transmission loss and wavelength in the wavelengthband of 1.53 μm to 1.57 μm in the whole optical transmission lineprovided with the dispersion-compensating module 10.

As shown in FIGS. 2A and 2B, each of the optical transmission line 2 anddispersion-compensating optical fiber 11 has a transmission loss whichbecomes smaller as the wavelength is longer in general in the wavelengthband of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. Bycontrast, as shown in FIG. 2C, the transition-metal-element-dopedoptical fiber 12 is a single-mode optical fiber whose core region isdoped with Co element at a concentration of about 10 ppm, which isdesigned such that its transmission loss becomes greater as thewavelength is longer, so as to be able to compensate forwavelength-dependent loss deviations in view of the loss slopes of theoptical transmission line 2 and dispersion-compensating optical fiber11.

Therefore, as shown in FIG. 2D, the total loss in thedispersion-compensating module 10 is the sum of respective losses in thedispersion-compensating optical fiber 11 and thetransition-metal-element-doped optical fiber 12, and becomes greater asthe wavelength is longer in the wavelength band of 1.53 μm to 1.57 μm,thus yielding a positive loss slope. As shown in FIG. 2E, the total lossin the optical transmission line 2 and the dispersion-compensatingmodule 10 is the sum of the irrespective losses, and yields a deviationof 0.1 dB or less in the wavelength band of 1.53 μm to 1.57 μm, wherebyits wavelength dependence is weaker than that of the loss deviation ofeach constituent.

FIGS. 3A and 3B are graphs showing the loss wavelength characteristic ofa standard SMF having a zero-dispersion wavelength in a 1.3-μmwavelength band. The graph of FIG. 3A shows the loss characteristicwithin a wavelength range of 1200 μm to 1700 μm; whereas the graph ofFIG. 3B enlarges a part of FIG. 3A, so as to show the losscharacteristic within a wavelength range of 1480 μm to 1620 μm. This SMFhas a stepped index type refractive index profile, whose core region isdoped with Ge element while silica is used as a base. As shown in thesegraphs, the loss in this SMF per unit length (km) varies about 0.007dB/km between wavelengths of 1530 nm and 1570 nm. In the wavelength bandhaving a width of 40 nm (=1570−1530), the loss slope of the SMF per unitlength is approximately −0.007/40=−0.000175 dB/nm/km (whereby the losson the longer wavelength side tends to become smaller).

FIGS. 4A and 4B are graphs showing an example of the loss wavelengthcharacteristic of a loss-equalizing optical fiber whose core region isdoped with Co element. The graph of FIG. 4A shows the losscharacteristic within a wavelength range of 600 μm to 1800 μm; whereasthe graph of FIG. 4B enlarges a part of FIG. 4A, so as to show the losscharacteristic within a wavelength range of 1500 μm to 1600 μm. Thisloss-equalizing optical fiber has a stepped index type refractive indexprofile, whose core region is doped with Co element while silica is usedas a base. As shown in these graphs, the loss slope of thisloss-equalizing optical fiber is positive in the wavelength band of 1.53μm to 1.57 μm. This loss slope can be adjusted by the amount of additionof Co element and the like.

When the SMF is used as the optical transmission line 2, the loss slopeof the optical transmission line 2 in the wavelength band of 1.53 μm to1.57 μm is negative as mentioned above. Therefore, if the loss slope ofthe whole dispersion-compensating module 10 is set positive, then thetotal loss in the optical transmission line 2 anddispersion-compensating module 10 can fall within an appropriate range.

Specifically, since the SMF employed as the optical transmission line 2has a loss slope per unit length of about −0.000175(dB/nm/km=dB/(nm·km)) in the wavelength band of 1.53 μm to 1.57 μm,letting L (km) be the fiber length of the optical transmission line 2,the loss slope (dB/nm) of the whole dispersion-compensating module 10 inthe wavelength band of 1.53 μm to 1.57 μm is ideally a value which isgreater than 0 but not greater than 0.000175×L.

The loss slope of the loss-equalizing optical fiber (loss-equalizingdevice) is set to an appropriate value by adjusting the amount ofaddition of Co element or the like, such that, while the loss slope ofthe dispersion-compensating optical fiber (dispersion-compensatingdevice) is taken into consideration, the loss slope of the wholedispersion-compensating module 10 falls within the range mentionedabove. In practice, however, the manufacturing error of theloss-equalizing optical fiber must be taken into consideration, wherebythe loss slope value S (dB/nm) of the whole dispersion-compensatingmodule 10 becomes 0<S≦0.000175×L+α where α is the absolute value ofmanufacturing error of the loss-equalizing optical fiber, which isspecifically about 000 dB/nm.

When the loss slope of the loss-equalizing optical fiber is controlledas such, the loss deviations among individual wavelengths occurring inthe optical transmission line 2 exceeding, for example, 80 km and thedispersion-compensating optical fiber can fall within an appropriaterange in the dispersion-compensating module 10 as a whole.

Second Embodiment

A second embodiment of the dispersion-compensating module according tothe present invention will now be explained. FIG. 5 is a view showing aschematic configuration of the dispersion-compensating module accordingto the second embodiment. This drawing shows, in addition to thedispersion-compensating module 20 according to this embodiment, arepeater 1 disposed upstream of the dispersion-compensating module 20,and an optical transmission line 2 between the repeater 1 and thedispersion-compensating module 20.

The dispersion-compensating module 20 according to this embodiment hasan input end 20 a and an output end 20 b, and is disposed while in astate where a dispersion-compensating device and a loss-equalizingdevice are optically connected to each other in the optical path betweenthe input end 20 a and the output end 20 b. In particular, thedispersion-compensating module 20 is constituted by adispersion-compensating optical fiber 21, as the dispersion-compensatingdevice, and an optical fiber 23 formed with a long-period fiber grating22, as the loss-equalizing device, which are connected to each other byfusion at a connecting portion 24. The optical fiber 23 is preferably anSMF or dispersion-compensating optical fiber having a zero-dispersionwavelength in a 1.3-μm wavelength band.

The dispersion-compensating optical fiber 21 is an optical device forcompensating for the chromatic dispersion in the WDM signal wavelengthband of the optical transmission line into which thedispersion-compensating module 20 is inserted. The long-period fibergrating 22 is obtained when a refractive index change having apredetermined period is generated in at least a core region of theoptical fiber 23, in which the period of refractive index change is along period on the order of several hundreds of micrometers, whereby thecore-mode light propagating through the core region and thecladding-mode light radiated to the cladding region are coupledtogether. By appropriately selecting the period of refractive indexchange and length, the long-period fiber grating 22 is designed suchthat, for example, the transmission loss at a wavelength of 1520 nm isminimized, while the transmission loss at a wavelength of 1570 nm ismaximized, whereby wavelength-dependent loss deviations of the opticaltransmission line 2 and the dispersion-compensating optical fiber 21 arecompensated for.

Therefore, the wavelength dependence of the total loss in the opticaltransmission line 2 and dispersion-compensating module 20 is weaker thanthat of the loss deviation in each of the dispersion-compensatingoptical fiber 21 and long-period fiber grating 22. When the long-periodfiber grating 22 is thus used as the loss-equalizing device, lossdeviations among individual signal light components can fall within anappropriate range without greatly decreasing the transmission loss inthe whole dispersion-compensating module 20. Also, desirabletransmission characteristics can easily be obtained in a wide wavelengthband. Here, the long-period fiber grating 22 is an optical componentwhich is clearly distinguished from a short-period fiber grating whichreflects only a signal light component having a predetermined wavelength(see U.S. Pat. No. 5,703,978).

Since graphs showing the relationships between transmission loss andwavelength in the dispersion-compensating module 20 according to thesecond embodiment are similar to FIGS. 2A to 2E, operations of thedispersion-compensating module 20 according to this embodiment will beexplained with reference to these graphs.

As shown in FIGS. 2A and 2B, each of the optical transmission line 2 anddispersion-compensating optical fiber 21 has a transmission loss whichbecomes smaller as the wavelength is longer in general in the wavelengthband of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. Bycontrast, as shown in FIG. 2C, the long-period fiber grating 22, whichis the loss-equalizing device, is designed such that its transmissionloss becomes greater as the wavelength is longer, so as to be able tocompensate for wavelength-dependent loss deviations in view of the lossslopes of the optical transmission line 2 and dispersion-compensatingoptical fiber 21.

Therefore, as shown in FIG. 2D, the total loss in thedispersion-compensating module 20 is the sum of respective losses in thedispersion-compensating optical fiber 21 and the long-period grating 22,and becomes greater as the wavelength is longer in the wavelength bandof 1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown inFIG. 2E, the total loss in the optical transmission line 2 and thedispersion-compensating module 20 is the sum of their respective losses,and yields a deviation of 0.1 dB or less in the wavelength band of 1.53μm to 1.57 μm.

FIG. 6 is a graph showing an example of the loss wavelengthcharacteristic of a long-period fiber grating. For making thislong-period fiber grating, a silica-based optical fiber having a steppedindex type refractive index profile, whose core region is doped with Geelement, is irradiated with ultraviolet rays through anintensity-modulating mask, so as to generate a refractive indexmodulation in the core region. As shown in this graph, the loss slope ofthe long-period fiber grating is positive in the wavelength band of 1.53μm to 1.57 μm. This loss slope can be adjusted by the period ofrefractive index change and the length.

As mentioned in the foregoing, when the SMF is used as the opticaltransmission line 2, the loss slope of the optical transmission line 2in the wavelength band of 1.53 μm to 1.57 μm is negative in thisembodiment as well. Therefore, if the loss slope of the wholedispersion-compensating module 20 is set positive, then the total lossin the optical transmission line 2 and dispersion-compensating module 20can fall within an appropriate range.

Also, since the SMF employed as the optical transmission line 2 has aloss slope per unit length (km) of about −0.000175 (dB/nm/km=dB/(nm km))in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be thefiber length of the optical transmission line 2, and α be the absolutevalue of a permissible manufacturing error, the loss slope (dB/nm) ofthe whole dispersion-compensating module 20 in the wavelength band of1.53 μm to 1.57 μm is preferably a value which is greater than 0 but notgreater than 0.000175×L+α.

Here, the loss slope of the long-period grating 22 is adjusted byappropriately setting the grating period and length such that the lossslope of the whole dispersion-compensating module 20 falls within therange mentioned above in view of the loss slope of thedispersion-compensating optical fiber 21.

Third Embodiment

A third embodiment of the dispersion-compensating module according tothe present invention will now be explained. FIG. 7 is a view showing aschematic configuration of the dispersion-compensating module accordingto the third embodiment. This drawing shows, in addition to thedispersion-compensating module 30 according to this embodiment, arepeater 1 disposed upstream of the dispersion-compensating module 30,and an optical transmission line 2 between the repeater 1 and thedispersion-compensating module 30.

The dispersion-compensating module 30 according to this embodiment hasan input end 30 a and an output end 30 b, and is disposed while in astate where a dispersion-compensating device and a loss-equalizingdevice are optically connected to each other in the optical path betweenthe input end 30 a and the output end 30 b. In particular, thedispersion-compensating module 30 is constituted by adispersion-compensating optical fiber 31, as the dispersion-compensatingdevice, and a long-period fiber grating 32, as the loss-equalizingdevice, directly formed in the dispersion-compensating optical fiber 31.

The dispersion-compensating optical fiber 31 is an optical device forcompensating for the chromatic dispersion in the WDM signal wavelengthband of the optical transmission line into which thedispersion-compensating module 30 is inserted. The long-period fibergrating 32 is obtained when a refractive index change having apredetermined period is generated in at least a core region of thedispersion-compensating optical fiber 31, in which the period ofrefractive index change is a long period on the order of severalhundreds of micrometers, whereby the core-mode light propagating throughthe core region and the cladding-mode light radiated to the claddingregion are coupled together. By appropriately selecting the period ofrefractive index change and the length, the long-period fiber grating 32is designed such that, for example, the transmission loss at awavelength of 1520 nm is minimized, while the transmission loss at awavelength of 1570 nm is maximized, whereby wavelength-dependent lossdeviations of the optical transmission line 2 anddispersion-compensating optical fiber 31 are compensated for.

Therefore, the total loss in the optical transmission line 2 anddispersion-compensating module 30 is the sum of the transmission loss inthe optical transmission line 2, the original transmission loss in thedispersion-compensating optical fiber 31, and the transmission loss inthe formed long-period fiber grating 32, thereby weakening thewavelength dependence as a whole. When the long-period fiber grating 32is thus used as the loss-equalizing device, loss deviations amongindividual signal light components can fall within an appropriate rangewithout greatly decreasing the transmission loss in the wholedispersion-compensating module 30. Also, desirable loss characteristicscan easily be obtained in a wide wavelength band. Further, in the thirdembodiment, since the long-period fiber grating 32, as theloss-equalizing device, is directly formed in thedispersion-compensating optical fiber 31, there is no connecting portionwhich may yield a loss, whereby it is unnecessary to take account of theinfluence of the loss in the connecting portion.

FIGS. 8A to 8D are graphs showing relationships between transmissionloss and wavelength in the dispersion-compensating module 30 accordingto the third embodiment. FIG. 8A is a graph showing the relationshipbetween transmission loss and wavelength in a wavelength band of 1.53 μmto 1.57 μm in the optical transmission line 2 employing the SMF. FIG. 8Bis a graph showing the relationship between transmission loss andwavelength in the wavelength band of 1.53 μm to 1.57 μm in thedispersion-compensating optical fiber 31 before the long-period fibergrating 32 is formed. FIG. 8C is a graph showing the relationshipbetween transmission loss and wavelength in the wavelength band of 1.53μm to 1.57 μm in the dispersion-compensating optical fiber 31 after thelong-period fiber grating 32 is formed, i.e., the relationship betweentransmission loss and wavelength in the wavelength band of 1.53 μm to1.57 μm in the dispersion-compensating module 30. FIG. 8D is a graphshowing the total relationship between transmission loss and wavelengthin the wavelength band of 1.53 μm to 1.57 μm in the optical transmissionline 2 and dispersion-compensating module 30.

As shown in FIG. 8A, the optical transmission line 2 has a transmissionloss which becomes smaller as the wavelength is longer in general in thewavelength band of 1.53 μm to 1.57 μm, thus yielding a negative lossslope. Also, as shown in FIG. 8B, the dispersion-compensating opticalfiber 31 before the formation of the long-period fiber grating 32 has atransmission loss which becomes smaller as the wavelength is longer ingeneral in the wavelength band of 1.53 μm to 1.57 μm, thus yielding anegative loss slope.

On the other hand, the long-period fiber grating 32 has a loss whichbecomes greater as the wavelength is longer, thereby compensating forthe original loss deviations among individual wavelengths of the opticaltransmission line 2 and dispersion-compensating optical fiber 31. Asshown in FIG. 8C, the total loss in the dispersion-compensating opticalfiber 31 formed with the long-period fiber grating 32, i.e., the wholedispersion-compensating module 30, is the sum of the originaltransmission loss in the dispersion-compensating optical fiber 31 andthe transmission loss in the long-period fiber grating 32, and has apositive loss slope in the wavelength band of 1.53 μm to 1.57 μm. Asshown in FIG. 8D, the total loss in the optical transmission line 2 andthe dispersion-compensating module 30 is the sum of their respectivelosses, and yields a deviation of 0.1 dB or less in the wavelength bandof 1.53 μm to 1.57 μm.

As mentioned in the foregoing, when the SMF is used as the opticaltransmission line 2, the loss slope of the optical transmission line 2in the wavelength band of 1.53 μm to 1.57 μm is negative in thisembodiment as well. Therefore, if the loss slope of the wholedispersion-compensating module 30 is set positive, then the total lossin the optical transmission line 2 and dispersion-compensating module 30can fall within an appropriate range.

Also, since the SMF employed as the optical transmission line 2 has aloss slope per unit length (km) of about −0.000175 (dB/nm/km=dB/(nm·km))in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be thefiber length of the optical transmission line 2, and a be the absolutevalue of a permissible manufacturing error, the loss slope (dB/nm) ofthe whole dispersion-compensating module 30 in the wavelength band of1.53 μm to 1.57 μm is preferably a value which is greater than 0 but notgreater than 0.000175×L+α.

Here, the loss slope of the whole dispersion-compensating module 30 isadjusted by appropriately setting the grating period and length of thelong-period grating 32.

Fourth Embodiment

A fourth embodiment of the dispersion-compensating module according tothe present invention will now be explained. FIG. 9 is a view showing aschematic configuration of the dispersion-compensating module accordingto the fourth embodiment. This drawing shows, in addition to thedispersion-compensating module 40 according to this embodiment, arepeater 1 disposed upstream of the dispersion-compensating module 40,and an optical transmission line 2 between the repeater 1 and thedispersion-compensating module 40.

The dispersion-compensating module 40 according to this embodiment isconstituted by a dispersion-compensating optical fiber 41, as adispersion-compensating device, and a single-mode optical fiber 42 whichare connected to each other by fusion at a fused portion 43. In thisconfiguration, the dispersion-compensating optical fiber 41 is anoptical device which compensates for the chromatic dispersion in thesignal light wavelength band of the optical transmission line into whichthe dispersion-compensating module 40 is inserted. Though the fusedportion 43 generates a loss, its wavelength characteristic variesdepending on fusion conditions such as the heating temperature uponconnecting by fusion and the amount of intrusion of the fiber, wherebythe wavelength dependence of transmission loss in the fused portion 43can be adjusted by appropriately setting these fusion conditions.

FIGS. 10A to 10C are views showing specific examples of thedispersion-compensating module according to the fourth embodiment. Aspecific structure of the fused portion 43 can be realized when, asshown in FIG. 10A for example, the core region 41 a of thedispersion-compensating optical fiber 41 and the core region 42 a of thesingle-mode optical fiber 42 are fused together while their optical axesAX1, AX2 are shifted from each other by a predetermined distance D. Itcan also be realized when, as shown in FIG. 10B, thedispersion-compensating optical fiber 41 and the single-mode opticalfiber 42 are connected to each other by fusion while each of the coreregion 41 b of the dispersion-compensating optical fiber 41 and the coreregion 42 b of the single-mode optical fiber 42 is minutely bent.Further, as shown in FIG. 10C, the core region 41 c of thedispersion-compensating optical fiber 41 and the core region 42 c of thesingle-mode optical fiber 42 may be configured so as to expand theirdiameters toward the fused portion 43. These specific examples can becombined. For example, in the fused portion 43, the radius of bend ofthe core region may be expanded, or structures of bending the coreregion may be combined together. In each of these cases, the deviationof total loss in the optical transmission line 2 anddispersion-compensating module 40 can be kept at 0.1 dB or less in thewavelength band of 1.53 μm to 1.57 μm.

Since graphs showing the relationships between transmission loss andwavelength in the dispersion-compensating module 40 according to thefourth embodiment are similar to FIGS. 2A to 2E, operations of thedispersion-compensating module 40 will be explained with reference tothese graphs.

As shown in FIGS. 2A and 2B, each of the optical transmission line 2 anddispersion-compensating optical fiber 41 has a transmission loss whichbecomes smaller as the wavelength is longer in general in the wavelengthband of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. Bycontrast, as shown in FIG. 2C, one of the amount of shift of opticalaxes, amount of bending of optical axes, and expanded core diameter ofthe fused portion 43, which is the loss-equalizing device, is designedsuch that its transmission loss becomes greater as the wavelength islonger, so as to be able to effectively compensate forwavelength-dependent loss deviations of the optical transmission line 2and dispersion-compensating optical fiber 41.

Therefore, as shown in FIG. 2D, the total loss in thedispersion-compensating module 40 is the sum of respective losses in thedispersion-compensating optical fiber 41 and the fused portion 43, andbecomes greater as the wavelength is longer in the wavelength band of1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown inFIG. 2E, the total loss in the optical transmission line 2 and thedispersion-compensating module 40 is the sum of their respective losses,and yields a deviation of 0.1 dB or less in the wavelength band of 1.53μm to 1.57 μm.

FIG. 11 is a graph showing an example of the loss wavelengthcharacteristic of the fused portion. As shown in this graph, the lossslope of this fused portion is positive in the wavelength band of 1.53μm to 1.57 μm. This loss slope can be adjusted by the amount of shift ofoptical axes, the amount of bending of optical axes, and the expandedcore diameter.

As mentioned in the foregoing, when the SMF is used as the opticaltransmission line 2, the loss slope of the optical transmission line 2in the wavelength band of 1.53 μm to 1.57 μm is negative in thisembodiment as well. Therefore, if the loss slope of the wholedispersion-compensating module 40 is set positive, then the total lossin the optical transmission line 2 and the dispersion-compensatingmodule 40 can fall within an appropriate range.

Also, since the SMF employed as the optical transmission line 2 has aloss slope per unit length (km) of about −0.000175 dB/nm/km in thewavelength band of 1.53 μm to 1.57 μm, letting L (km) be the fiberlength of the optical transmission line 2, and α be the absolute valueof a permissible manufacturing error, the loss slope (dB/nm) of thewhole dispersion-compensating module 40 in the wavelength band of 1.53μm to 1.57 μm is preferably a value which is greater than 0 but notgreater than 0.000175×L+α.

Though the fusion connection between the dispersion-compensating opticalfiber 41 and the SMF 42 is explained in the fourth embodiment, theconfiguration of the fused portion 43 should not be restricted thereto.For example, an SMF may be used in place of the dispersion-compensatingoptical fiber 41, and a dispersion-compensating optical fiber or otheroptical fibers may be used in place of the SMF 42. In any case, if thewavelength dependence of transmission loss in the fused portiontherebetween is adjusted, then the wavelength dependence of the totalloss in the optical transmission line and dispersion-compensating modulecan be weakened.

Fifth Embodiment

A fifth embodiment of the dispersion-compensating module according tothe present invention will now be explained. FIG. 12 is a view showing aschematic configuration of the dispersion-compensating module accordingto the fifth embodiment. This drawing shows, in addition to thedispersion-compensating module 50 according to this embodiment, arepeater 1 disposed upstream of the dispersion-compensating module 50,and an optical transmission line 2 between the repeater 1 and thedispersion-compensating module 50.

The dispersion-compensating module 50 according to this embodiment isdisposed in a state where a dispersion-compensating device and aloss-equalizing device are optically connected to each other in theoptical path between an input end 50 a and an output end 50 b.Specifically, this embodiment comprises a dispersion-compensatingoptical fiber 51 as the dispersion-compensating device and a fiberfusion type coupler (WDM coupler) 52 as the loss-equalizing device. TheWDM coupler 52 preferably has a polarization-dependent loss (PDL) of 0.2dB or less.

The dispersion-compensating optical fiber 51 is an optical device whichcompensates for the chromatic dispersion in the WDM signal wavelengthband of the optical transmission line into which thedispersion-compensating module 50 is inserted. The WDM coupler 52 isobtained by fusing together two optical fibers disposed in parallel;and, its fusion conditions and coupling length are appropriatelyselected such that, for example, the transmission loss at a wavelengthof 1520 nm is minimized, while the transmission loss at a wavelength of1570 nm is maximized, whereby wavelength-dependent loss deviations ofthe optical transmission line 2 and dispersion-compensating opticalfiber 51 are compensated for. As a consequence, the wavelengthdependence of the total loss in the optical transmission line 2 anddispersion-compensating module 50 is weakened as a whole.

Since graphs showing the relationships between transmission loss andwavelength in the dispersion-compensating module 50 according to thefifth embodiment are similar to FIGS. 2A to 2E, operations of thedispersion-compensating module 50 will be explained with reference tothese graphs.

As shown in FIGS. 2A and 2B, each of the optical transmission line 2 anddispersion-compensating optical fiber 51 has a transmission loss whichbecomes smaller as the wavelength is longer in general in the wavelengthband of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. Bycontrast, as shown in FIG. 2C, the fusion conditions and coupling lengthof the WDM coupler 52, which is the loss-equalizing device, are designedsuch that its transmission loss becomes greater as the wavelength islonger, so as to be able to effectively compensate forwavelength-dependent loss deviations of the optical transmission line 2and dispersion-compensating optical fiber 51.

Therefore, as shown in FIG. 2D, the total loss in thedispersion-compensating module 50 is the sum of respective losses in thedispersion-compensating optical fiber 51 and the WDM coupler 52, andbecomes greater as the wavelength is longer in the wavelength band of1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown inFIG. 2E, the total loss in the optical transmission line 2 and thedispersion-compensating module 50 is the sum of their respective losses,and yields a deviation of 0.1 dB or less in the wavelength band of 1.53μm to 1.57 μm.

As mentioned in the foregoing, when the SMF is used as the opticaltransmission line 2, the loss slope of the optical transmission line 2in the wavelength band of 1.53 μm to 1.57 μm is negative in thisembodiment as well. Therefore, if the loss slope of the wholedispersion-compensating module 50 is set positive, then the total lossin the optical transmission line 2 and dispersion-compensating module 50can fall within an appropriate range.

Also, since the SMF employed as the optical transmission line 2 has aloss slope per unit length (km) of about −0.000175 (dB/nm/km=db/(nm·km))in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be thefiber length of the optical transmission line 2, and a be the absolutevalue of a permissible manufacturing error, the loss slope (dB/nm) ofthe whole dispersion-compensating module 50 in the wavelength band of1.53 μm to 1.57 μm is preferably a value which is greater than 0 but notgreater than 0.000175×L+α.

Here, the loss slope of the WDM coupler 52 is adjusted by appropriatelysetting the fusion conditions and coupling length such that the lossslope of the whole dispersion-compensating module 50 falls within therange mentioned above in view of the loss slope of thedispersion-compensating optical fiber 51.

Sixth Embodiment

A sixth embodiment of the dispersion-compensating module according tothe present invention will now be explained. FIG. 13 is a view showing aschematic configuration of the dispersion-compensating module accordingto the sixth embodiment. This drawing shows, in addition to thedispersion-compensating module 60 according to this embodiment, arepeater 1 disposed upstream of the dispersion-compensating module 60,and an optical transmission line 2 between the repeater 1 and thedispersion-compensating module 60.

The dispersion-compensating module 60 according to this embodiment hasan input end 60 a and an output end 60 b, and is disposed while in astate where a dispersion-compensating device and a loss-equalizingdevice are optically connected to each other in the optical path betweenthe input end 60 a and the output end 60 b. In particular, thedispersion-compensating module 60 is constituted by adispersion-compensating optical fiber 61, as the dispersion-compensatingdevice, and an optical fiber 63 having a bent portion 62, as theloss-equalizing device, which are connected to each other by fusion at aconnecting portion 64. The optical fiber 63 is preferably an SMF ordispersion-compensating optical fiber having a zero-dispersionwavelength in a 1.3-μm wavelength band. It is also preferable that theoptical fiber 63 be common with the dispersion-compensating opticalfiber 61.

The dispersion-compensating optical fiber 61 is an optical device forcompensating for the chromatic dispersion in the WDM signal wavelengthband of the optical transmission line into which thedispersion-compensating module 60 is inserted. In the bent portion 62, aplurality of parts of the optical fiber 63 are bent at a predeterminedcurvature over a predetermined length. The bent portion 62 is designedby appropriately selecting the length and curvature such that, forexample, the transmission loss at a wavelength of 1520 nm is minimized,while the transmission loss at a wavelength of 1570 nm is maximized,whereby wavelength-dependent loss deviations of the optical transmissionline 2 and dispersion-compensating optical fiber 61 are compensated for.As a consequence, the wavelength dependence of the total loss in theoptical transmission line 2 and dispersion-compensating module 60 isweaker than that of the respective loss deviations of thedispersion-compensating optical fiber 61 and the bent portion 62.

Since graphs showing the relationships between transmission loss andwavelength in the dispersion-compensating module 60 according to thesixth embodiment are similar to FIGS. 2A to 2E, operations of thedispersion-compensating module 60 will be explained with reference tothese graphs.

As shown in FIGS. 2A and 2B, each of the optical transmission line 2 anddispersion-compensating optical fiber 61 has a transmission loss whichbecomes smaller as the wavelength is longer in general in the wavelengthband of 1.53 μm to 1.57 μm, thus yielding a negative loss slope. Bycontrast, as shown in FIG. 2C, the bent portion 60, which is theloss-equalizing device, is designed such that its transmission lossbecomes greater as the wavelength is longer, so as to be able toeffectively compensate for wavelength-dependent loss deviations of theoptical transmission line 2 and dispersion-compensating optical fiber61.

Therefore, as shown in FIG. 2D, the total loss in thedispersion-compensating module 60 is the sum of respective losses in thedispersion-compensating optical fiber 61 and the bent portion 62, andbecomes greater as the wavelength is longer in the wavelength band of1.53 μm to 1.57 μm, thus yielding a positive loss slope. As shown inFIG. 2E, the total loss in the optical transmission line 2 and thedispersion-compensating module 60 is the sum of their respective losses,and yields a deviation of 0.1 dB or less in the wavelength band of 1.53μm to 1.57 μm.

As mentioned in the foregoing, when the SMF is used as the opticaltransmission line 2, the loss slope of the optical transmission line 2in the wavelength band of 1.53 μm to 1.57 μm is negative in thisembodiment as well. Therefore, if the loss slope of the wholedispersion-compensating module 60 is set positive, then the total lossin the optical transmission line 2 and dispersion-compensating module 60can fall within an appropriate range.

Also, since the SMF employed as the optical transmission line 2 has aloss slope per unit length (km) of about −0.000175 (dB/nm/km=db/(nm·km))in the wavelength band of 1.53 μm to 1.57 μm, letting L (km) be thefiber length of the optical transmission line 2, and a be the absolutevalue of a permissible manufacturing error, the loss slope (dB/nm) ofthe whole dispersion-compensating module 60 in the wavelength band of1.53 μm to 1.57 μm is preferably a value which is greater than 0 but notgreater than 0.000175×L+α.

Here, the loss slope of the whole dispersion-compensating module 60 isset by appropriately setting the length and curvature of the bentportion 62 so as to adjust the loss slope of the bent portion 62.

In each of the above-mentioned first to sixth embodiments, either thedispersion-compensating device or the loss-equalizing device may bedisposed upstream of the other. In view of influences of nonlinearoptical phenomena (four-wave mixing in particular), however, it ispreferable that the loss-equalizing device be disposed upstream of thedispersion-compensating device. Namely, as a consequence, signal lightenters the dispersion-compensating device after incurring a loss due tothe loss-equalizing device, whereby nonlinear optical phenomena such asfour-wave mixing and the like are effectively restrained from occurring.

Seventh to Tenth Embodiments

FIG. 14 is a view showing a schematic common configuration of seventh totenth embodiments of the dispersion-compensating module according to thepresent invention. This drawing shows, in addition to thedispersion-compensating module 70 according to each of seventh to tenthembodiments, a repeater 1 disposed upstream of thedispersion-compensating module 70, and an optical transmission line 2between the repeater 1 and the dispersion-compensating module 70.

The dispersion-compensating module 70 has an input end 70 a and anoutput end 70 b, and comprises a dispersion-compensating device 71 and aloss-equalizing device 72 optically connected to each other at aconnection portion 73. These devices 71 and 72 are disposed in theoptical path between the input end 70 a and the output end 70 b.

The dispersion-compensating device 71 is preferably adispersion-compensating optical fiber as an optical device whichcompensates for the chromatic dispersion in the WDM signal wavelengthband of the optical transmission line 2 into which thedispersion-compensating module 70 is inserted. On the other hand, theloss-equalizing device 72 has a loss wavelength characteristic adjustedso as to compensate for wavelength-dependent loss deviations of theoptical transmission line 2 and dispersion-compensating optical fiber asthe dispersion-compensating device 71. As a consequence, the total lossfluctuation in the signal wavelength band of the optical transmissionline 2 provided with the dispersion-compensating module 70 decreases.

In the dispersion-compensating module 70 according to the seventhembodiment, the loss-equalizing device 72 includes a slant type fibergrating. The slant type fiber grating 721, as shown in FIG. 15A,comprises a optical fiber and a grating formed in the optical fiberwhile being inclined at a predetermined angle with respect to an opticalaxis OP of the optical fiber. It is known that a loss of the slant typefiber grating 721 as the loss-equalizing device 72 has a wavelengthdependency as described in Isabelle Riant, et al. “36 NM AMPLIFIER GAINEQUALIZER BASED ON SLANTED BRAGG GRATING TECHNOLOGY FOR MULTICHANNELTRANSMISSION”, SubOptic 2001 International Convention, P.4.3.10. TheIsabelle reference teaches the use of the slant type fiber grating forEDFAs as a gain equalizer, but a positive loss slope of the wholedispersion-compensating module 70 can be realized by combining thedispersion-compensating optical fiber as the dispersion-compensatingdevice 71 and this slant type fiber grating 721 with desirably modifieddesign. The slant type fiber grating 721 in the seventh embodimentincludes a single slanted grating and a combination of a plurality ofslanted gratings.

Next, the loss-equalizing device 72 in the dispersion-compensatingmodule according to the eighth embodiment includes a dielectricmultilayered filter 722 shown in FIG. 15B. A loss wavelength dependencyof the dielectric multilayered filter 722 can be adjusted by adjusting athickness, a refractive index of layer, the number of layer thereof.Therefore, a positive loss slope of the dispersion-compensating module70 can be realized by combining the dispersion-compensating opticalfiber as the dispersion-compensating device 71 and this dielectricmultilayered filter 722.

Furthermore, the loss-equalizing device 72 of FIG. 14 may has astructure with a variable loss wavelength characteristic. Theloss-equalizing device 72 in the ninth embodiment, as sown in FIG. 15C,includes a variable loss-equalizing device having a planar waveduide(see Hitoshi Hatayama, et al., “Low loss variable attenuation slopecompensator with high slope linearity based on planar lightwavecircuit”, ECOC 2000, 26^(th) European Conference on OpticalCommunication, pp.287-288). The variable loss-equalizing device 723 ofFIG. 15C comprises a substrate 723 a, a waveguide 723 b formed on thesubstrate 723 a, and a heater 723 c. As be understood from the Hitoshireference, the loss-equalizing device 723 can modify it's loss slope.For example, the reference shows the loss wavelength characteristic from1570 nm to 1605 nm. Therefore, a positive loss slope of the wholedispersion-compensating module 72 according to the ninth embodiment canbe realized by desirably changing a design of this variableloss-equalizing device 723 and by combining the dispersion-compensatingoptical fiber as the dispersion-compensating device 71 and this variableloss-equalizing device 723.

Further, the loss-equalizing device 72 may includes a variableloss-equalizing device with MEMS (Micro Electro Mechanical Systems).FIGS. 15D to 15F show a configuration of the variable loss-equalizingdevice in tenth embodiment. The loss-equalizing device is shown in JamesA. Walker, “Telecommunications Applications of MEMS”, mstnews 3/00, pp.6-9 and J. E. Ford, et al., “PASSBAND-FREE DYNAMIC WDM EQUALIZATION”,ECOC'98, 20-24 Sep. 1998, pp. 317-318. In the loss-equalizing device ofthe tenth embodiment, input light is demultiplexed at the grating, andthe reflected light from the grating is introduced to the device planeof the optical MEMS device through λ/4 plate and focus lens. On thedevice plane of the optical MEMS device, a plurality of mirror devices,as shown in FIGS. 15E and 15F, are provided. Each of these mirrordevices can change it's reflectance on the based on an added voltage.The light from the device plate is multiplexed at the grating andoutputted through the mirror and collimator. By adjusting voltages addedto the plurality of mirror devices respectively, a loss wavelengthcharacteristic of components in the demultiplexed wavelength range canbe changed, and therefore the combination of the dispersion-compensatingoptical fiber as the dispersion-compensating device 71 and this variableloss-equalizing device of FIGS. 15D to 15F can make a loss slope of thewhole dispersion-compensating module 70 positive.

According to the present invention, as explained in the foregoing, thedispersion of the optical transmission line in the wavelength band of1.53 μm to 1.57 μm is compensated for by the dispersion-compensatingdevice, whereas the loss deviations of the optical transmission line anddispersion-compensating device in the wavelength band of 1.53 μm to 1.57μm are compensated for by the loss-equalizing device. Namely, not onlythe dispersion of the optical transmission line is compensated for, butalso the wavelength dependence of the total loss in the opticaltransmission line and dispersion-compensating module is weaker. As aconsequence, the intensity level deviations among individual wavelengthsof the WDM signal reaching the receiving station are small, and eachwavelength component of the WDM signal reaches the receiving stationwith a sufficient intensity level and SN ratio, whereby no receptionerror occurs in the receiving station.

In particular, since the dispersion-compensating module as a whole has apositive loss slope in the wavelength band of 1.53 μm to 1.57 μm withrespect to an optical transmission line made of an SMF having azero-dispersion wavelength in the wavelength band of 1.3 μm (whereas theoptical transmission line has a negative loss slope in the wavelengthband of 1.53 μm to 1.57 μm), the total loss in the optical transmissionline and dispersion-compensating module can fall within an appropriaterange.

Also, letting L (km) be the fiber length of the optical transmissionline, and a be the absolute value of a permissible manufacturing error,the loss slope (dB/nm) of the dispersion-compensating module in thewavelength band of 1.53 μm to 1.57 μm is a value which is greater than 0but not greater than 0.000175×L+α. Since the optical transmission linemade of an SMF having a zero-dispersion wavelength in the wavelengthband of 1.3 m is about −0.000175 dm/nm/km in the wavelength band of 1.53μm to 1.57 μm; even if the loss slope of the optical transmission linein the wavelength band of 1.53 μm to 1.57 μm has a fluctuation, thetotal loss in the optical transmission line and dispersion-compensatingmodule can fall within an appropriate range when the loss slope of thewhole dispersion-compensating module is set to a value within the rangementioned above.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

1. A dispersion-compensating module to be installed at a predeterminedposition on an optical transmission line having a predetermined length,said dispersion-compensating module having a positive loss slope in a1.55-μm wavelength band and constituting a part of said opticaltransmission line, said dispersion-compensating module comprising: adispersion-compensating device for compensating for the dispersion ofsaid optical transmission line in the 1.55-μm wavelength band; and aloss-equalizing device for adjusting the total loss slope of saidoptical transmission line including said dispersion-compensating modulesuch that a loss deviation between individual signal wavelengths in the1.55-μm wavelength band caused by propagation in said opticaltransmission line and dispersion-compensating device falls within anappropriate range, wherein said loss-equalizing device includes a slanttype fiber grating.
 2. A dispersion-compensating module to be installedat a predetermined position on an optical transmission line having apredetermined length, said dispersion-compensating module having apositive loss slope in a 1.55-μm wavelength band and constituting a partof said optical transmission line, said dispersion-compensating modulecomprising: a dispersion-compensating device for compensating for thedispersion of said optical transmission line in the 1.55-μm wavelengthband; and a loss-equalizing device for adjusting the total loss slope ofsaid optical transmission line including said dispersion-compensatingmodule such that a loss deviation between individual signal wavelengthsin the 1.55-μm wavelength band caused by propagation in said opticaltransmission line and dispersion-compensating device falls within anappropriate range, wherein said loss-equalizing device includes adielectric multilayered filter.
 3. A dispersion-compensating module tobe installed at a predetermined position on an optical transmission linehaving a predetermined length, said dispersion-compensating modulehaving a positive loss slope in a 1.55-μm wavelength band andconstituting a part of said optical transmission line, saiddispersion-compensating module comprising: a dispersion-compensatingdevice for compensating for the dispersion of said optical transmissionline in the 1.55-μm wavelength band; and a loss-equalizing device foradjusting the total loss slope of said optical transmission lineincluding said dispersion-compensating module such that a loss deviationbetween individual signal wavelengths in the 1.55-μm wavelength bandcaused by propagation in said optical transmission line anddispersion-compensating device falls within an appropriate range,wherein said loss-equalizing device has a variable loss wavelengthcharacteristic.
 4. A dispersion-compensating module to be installed at apredetermined position on an optical transmission line having apredetermined length, said dispersion-compensating module having apositive loss slope in a 1.55-μm wavelength band and constituting a partof said optical transmission line, said dispersion-compensating modulecomprising: a dispersion-compensating device for compensating for thedispersion of said optical transmission line in the 1.55-μm wavelengthband; and a loss-equalizing device for adjusting the total loss slope ofsaid optical transmission line including said dispersion-compensatingmodule such that a loss deviation between individual signal wavelengthsin the 1.55-μm wavelength band caused by propagation in said opticaltransmission line and dispersion-compensating device falls within anappropriate range, wherein said loss-equalizing device includes avariable loss-equalizing device having a planar waveguide.
 5. Adispersion-compensating module to be installed at a predeterminedposition on an optical transmission line having a predetermined length,said dispersion-compensating module having a positive loss slope in a1.55-μm wavelength band and constituting a part of said opticaltransmission line, said dispersion-compensating module comprising: adispersion-compensating device for compensating for the dispersion ofsaid optical transmission line in the 1.55-μm wavelength band; and aloss-equalizing device for adjusting the total loss slope of saidoptical transmission line including said dispersion-compensating modulesuch that a loss deviation between individual signal wavelengths in the1.55-μm wavelength band caused by propagation in said opticaltransmission line and dispersion-compensating device falls within anappropriate range, wherein said loss-equalizing device includes avariable loss-equalizing device includes a variable loss-equalizingdevice with Micro Electro Mechanical Systems.